1. Technical Field
The invention relates generally to light-emitting devices and more particularly to light emitting devices with improved light extraction.
2. Description of the Prior Art
FIG. 1 depicts a prior art semiconductor light emitter 100 consisting of substrate 20, multi-layered structure 26, and top layer 24. Semiconductor light emitter 100 may be, for example, a light emitting diode (“LED”) or a semiconductor laser. An LED is a p-n junction device that is designed to convert an incoming flow of electric energy into an outgoing flow of electromagnetic radiation. An LED may emit electromagnetic radiation in ultraviolet, visible, or infrared regions of the electromagnetic spectrum. The visible LEDs are commonly used for illumination and displays, and also have applications as an information link between electronic instruments and their users. The infrared LEDs are useful in opto-isolators and optical-fiber communications. A semiconductor laser is constructed in a manner similar to an LED.
Multi-layered structure 26 includes but is not limited to lower confining layer 21, upper confining layer 23, and active layer 22 where photons are emitted. Upper confining layer 23 may include top layer 24. Where semiconductor light emitter 100 does not have a separate top layer 24, confining layer 23 is the top layer.
Confining layers 21, 23 and active layer 22 of multi-layered structure 26 are typically formed from III-V semiconductors, III-nitride semiconductors, and II-VI semiconductors. Top layer 24, which may be epitaxially grown on upper confining layer 23, is also typically a III-V semiconductor, III-nitride semiconductor, II-VI semiconductor, or an alloy thereof. However, top layer 24 can have a semiconductor alloy composition that is different from the material forming confining layer 21 or confining layer 23. It is preferable that top layer 24 is made of a material that has a bandgap greater than that of active layer 22 so as to be transparent to the light emitted by active layer 22. As used herein, the term “transparent” indicates that an optical element so described transmits light at the emission wavelengths of the particular semiconductor light emitter with less than about 50%, and preferably less than about 10%, single pass loss due to absorption or scattering. Top layer 24 can be a transparent substrate (a superstrate) wafer-bonded to the upper confining layer 23. Top layer 24 may also be the substrate on which the epitaxial layers have been grown.
Lower confining layer 21 and upper confining layer 23 are electrically coupled to active layer 22 and to contact 31 and contact 32. Typically, one confining layer is doped with donors to make an n-type confining layer, and the other confining layer is doped with acceptors to make a p-type confining layer. Thus, upon the application of suitable voltage across contacts 31 and 32, electrons from the n-type confining layer and the holes from the p-type confining layer combine in active layer 22 and emit light isotropically. Further details on semiconductor light emitter 100 as an LED are provided in U.S. Pat. No. 6,133,589 to Michael R. Krames, et. al. entitled “AlGaInN-based LED Having Thick Epitaxial Layer for Improved Light Extraction,” and U.S. Pat. No. 5,793,062 and U.S. Pat. No. 6,015,719 to Fred A. Kish, Jr. et. al., both of which are entitled “Transparent Substrate Light Emitting Diode with Directed Light Output.” All of these patents are herein incorporated by reference.
Semiconductor light emitter 100 may be LED 100. A problem with LEDs is a low light extraction efficiency. The low light extraction efficiency is caused by only a fraction (e.g., approximately 30% for an AlGaAs LED with a transparent substrate) of the light energy emitted by active layer 22 managing to escape LED 100. As a consequence of the low light extraction efficiency, only a fraction of the consumed electrical input results in useful externally observable light. Light extraction efficiency is defined as the ratio of the number of photons that escape the LED to the number of photons generated in the LED.
Path 3 of FIG. 1 shows the direction of a photon emitted from point source 27 of active layer 22. As shown by path 3, the absorptive property of contacts 31, 32 contribute to the low light extraction. The photon traveling along path 3 reflects off the inner surface of LED 100 and becomes absorbed by contact 31. Contacts 31 and 32 may be formed from metals such as gold, nickel, aluminum, titanium, chromium, platinum, palladium, and mixtures or alloys thereof.
Loss mechanisms responsible for the low light extraction efficiency include absorption within the semiconductor light emitter, reflection loss when light passes from one type of material to another material that has a different index of refraction, and total internal reflection followed by absorption within the light emitting device. Total internal reflection, however, prevents photons from escaping semiconductor light emitter 100 only when photons emitted by active layer 22 reach the interface of light emitting device 100 and the surrounding material at an angle greater than the critical angle (θc). Critical angle (shown in FIG. 1 as “θc”), as it relates to the present embodiment, is defined as θc=arcsin(nsurrounding/nLED) where nsurrounding and NLED indicate the refractive index of the material surrounding the light emitting device and the refractive index of the light emitting device, respectively. An LED is frequently encapsulated in epoxy, whose index of refraction (nepoxy) is around 1.5. For an LED made of one of the III-V semiconductor materials mentioned above, the index of refraction ranges from about 2.4 to about 4.1 at the typical LED emission wavelengths. Taking an average refractive index (nLED) to be approximately 3.5, a typical value of θc is approximately 25°. Thus, a photon emitted from a point source 27 in active layer 22 can escape LED 100 from any surface through an “escape cone” with a half-angle of 25°. Photons that strike the interface between LED 100 and the surrounding material outside the escape cone may be subjected to a series of internal reflections and become absorbed by, for example, the semiconductor layers (including active layer 22) or contacts 31 and 32. Thus, many of the photons that strike a surface at an angle greater than 25° to an axis normal to the surface do not escape the LED on the first pass. An LED with a higher light extraction efficiency that allows more of the emitted photons to escape the LED is needed.